This invention relates generally to a method for metallizing fluoropolymer substrates. More particularly, this invention relates to a new and improved method for metallizing fluoropolymer substrates which includes the step of bias sputtering in a mixture of ammonia/noble gas (e.g., argon).
Laminated electrical circuits having metal layers supported on fluoropolymeric substrate layers are known. However, it has proven difficult to produce a metal/fluoropolymer laminate with good adhesion of the metal layer to the polymer substrate. This is especially true when the metal layer is a thin directly deposited layer of less than a few microns. Often, this thin layer (e.g., copper-seed layers) of metal is deposited onto the fluoropolymer substrate by bias sputter deposition in an atmosphere of argon gas.
Several methods for improving the adhesion of a metal layer to a fluoropolymeric substrate are known. In one such method, a fluoropolymeric substrate may be etched using a known sodium/naphthalene pretreatment (commercially available under the tradename TETRA-ETCH by W. L. Gore & Associates) prior to lamination of the metal layer. Unfortunately, this sodium etching process presents several drawbacks in that the etching solution is very corrosive and results in a hazardous waste disposal problem. In addition, this etching process provides no improvement in the adhesion of metal film to extruded perfluorovinyl ether (PFA). This is a serious drawback to sodium/naphthalene pretreatment as PFA is a favored material for use in electrical circuit laminates.
Another method involves electroless metal deposition techniques for depositing the metal layer on the fluoropolymeric substrate, but this method requires the maintenance of wet plating baths and again generates hazardous wastes. High temperature fusion lamination of metal foil to a fluoropolymeric substrate is another process, but this process suffers from problems with regard to maintaining the dimensional stability of the laminate.
Still another method of improving the adhesion between a fluoropolymer substrate and a metal layer is disclosed in U.S. Pat. No. 4,731,156 to Montmarquet. The Montmarquet patent discloses a method wherein wettability of a fluoropolymer surface for later adhesion with a metal foil, substrate or the like is improved by plasma pretreating the fluoropolymer substrate in sealed chamber filled with a mixture of ammonia alone or in combination with oxygen, nitrogen or argon.
While the process of U.S. Pat. No. 4,731,156 leads to some improvement in adhesion, there nevertheless continues to be a need for even better adhesion between fluoropolymer substrates, particularly PFA films, and metallized layers. There also continues to be a need for a method for making a metal/fluoropolymer laminate having high adhesion between the metal and fluoropolymeric layers without generating a hazardous waste stream or compromising the dimensional stability of the laminate.